The fallacy of false dichotomy is committed when the arguer
claims that his conclusion is one of only two options, when in fact there
are other possibilities. The arguer then goes on to show that the 'only
other option' is clearly outrageous, and so his preferred conclusion must
be embraced.

Either you let me go to the Family Values Tour, or I'll
be miserable for the rest of my life. I know you don't want me to be miserable
for the rest of my life, so you should let me go to the concert.

Either you use Speed Stick deodorant, or you will stink
to high heaven. You don't want to stink, so you better by Speed Stick.

Either I keep smoking, or I'll get fat. I don't want to
get fat, so I better keep smoking.

Either we keep Charles Manson in jail, or we release him,
thus risking murder, carnage, and mayhem. We don't want murder, carnage,
or mayhem, so we had better keep him in jail.